In general, combustion devices, such as boilers and water heaters, used for heating or producing hot water are classified according to fuel into oil boilers, gas boilers, electric boilers, water heaters, etc. Various combustion devices are developed and appropriately used according to purposes thereof.
Of conventional combustion devices, a gas boiler and a water heater generally use a Bunsen burner or a premixed burner for burning gas fuel, and among them, the premixed burner realizes combustion by mixing gas and air at an optimum mixing ratio for combustion and supplying the mixture (air+gas) to a burner port.
Further, the performance of the combustion device is typically measured by the turn-down ratio (TDR). Here, the turn-down ratio means ‘the ratio of the maximum gas consumption to the minimum gas consumption’ in a gas combustion device in which the quantity of gas is variably controlled. For example, when the maximum gas consumption is set to 24,000 kcal/h and the minimum gas consumption is set to 8,000 kcal/h, the turn-down ratio (TDR) is 3:1. Here, the turn-down ratio (TDR) is typically limited by the capability of the combustion device to maintain a stable flame under the condition of minimum gas consumption.
In the case of a gas boiler and a water heater, the convenience thereof when using the device for heating or for producing hot water may be increased in proportion to the higher value of the turn-down ratio (TDR). In other words, when the turn-down ratio (TDR) is low (the case in which the minimum gas consumption is high) and the burner is operated in an area with a low load of heating or hot water, the combustion device may turn on and off frequently so that the deviation when controlling the temperature is increased and the durability of the device is reduced. To solve the above problems, various technologies have been developed to increase the turn-down ratios (TDR) of burners of combustion devices.
Gas supply valves used in the above-mentioned modulating burners are classified into an electric modulating gas valve that is largely controlled by an electric current and a pneumatic modulating gas valve that is controlled by an air pressure difference generated when air is supplied to a burner.
Here, the pneumatic modulating gas valve controls the quantity of gas supplied to the burner by using an air pressure difference generated when air required for combustion is supplied to the burner using a blower. Here, the air and gas required for combustion are mixed in a gas-air mixer and then supplied to the burner as mixed gas (a mixture of air and gas).
In the gas-air mixer of the gas burner using the pneumatic modulating gas valve, the turn-down ratio (TDR) is generally limited by the relationship between the gas consumption (Q) and the pressure difference (ΔP). In a fluid, the relationship between the flow rate and the pressure difference (ΔP) is expressed by the following equation.Q=k√{square root over (ΔP)}
That is, the pressure difference of a fluid must be increased four times in order to double flow rate thereof.
Accordingly, to set the turn-down ratio (TDR) to 3:1 for instance, the ratio of the pressure difference must be set to 9:1. Further, to set the turn-down ratio (TDR) to 10:1 for instance, the ratio of the pressure difference must be set to 100:1. However, it is impossible to infinitely increase the supplied pressure of gas.
To overcome the problems experienced by the fact that it is impossible to increase the supplied pressure of gas infinitely, a method of increasing the turn-down ratio (TDR) by dividing each path for supplying air and gas into two or more parts as shown in FIG. 1 and opening and closing each path for supplying gas to the burner was proposed.